The signal E1 of the GALILEO satellite navigation system is made up of a coherent sum of two signals. The first signal, called pilot signal, is modulated by a first spreading code and is used mainly to perform distance measurements between a satellite and a receiver of the signal in order to perform positioning computations. The second signal is a signal which conveys data. It is modulated by a second spreading code, each period of the code being associated with a symbol to be sent. A symbol is obtained by applying a binary modulation to the bit to be transmitted. The two spreading codes are different but of identical periods, for example, in the case of the signal E1, this period is equal to 4 ms.
Upon the reception of the signal, the purpose of the processing operations performed is notably to detect the start of a period of the spreading code of the pilot signal but also to demodulate the data of the second signal. For this, a correlation computation is performed between a local replica of the first spreading code with the signal. The result of the correlation is then integrated over a plurality of periods of the code (for example 25 periods) to counter the influence of the thermal noise and allow a correlation peak to be identified.
The result of the correlation is disturbed by two distinct sources of noise. Firstly, a thermal noise disturbs the signal in its transmission and affects the result of the correlation with the local code. One conventional means for countering the influence of the thermal noise consists in increasing the integration time. However, the presence of the data signal modulated with a second spreading code also disturbs the result of the correlation of the overall signal with the first spreading code because the intercorrelation between the pilot signal and the data signal is not zero. The impact of the level of intercorrelation between the two signals on the result of the correlation can be considerable in particular for the applications which require enhanced accuracy on the positioning information. Furthermore, the noise linked to the intercorrelation between the pilot signal and the data signal can become more influential than the thermal noise for high signal-to-noise ratios.
The known GNSS receivers more often than not use a high coherent integration time to counter the influence of the noise. Now, this solution does not make it possible to reduce the level of intercorrelation between the pilot signal and the data signal.